Field of the Invention
The present specification generally relates to light-diffusing optical fibers for use in illumination applications and, more specifically, to light-diffusing optical fibers capable of producing color movement along the length of the fiber.
Background
Optical fibers are used for a variety of applications where light needs to be delivered from a light source to a remote location. Optical telecommunication systems, for example, rely on a network of optical fibers to transmit light from a service provider to system end-users.
Telecommunication optical fibers are designed to operate at near-infrared wavelengths in the range from 800 nm to 1675 nm where there are only relatively low levels of attenuation due to absorption and scattering. This allows most of the light injected into one end of the fiber to exit the opposite end of the fiber with only insubstantial amounts exiting peripherally through the sides of the fiber.
Recently, however, there has been a growing need to have optical fibers that are less sensitive to bending than conventional fibers. This is because more and more telecommunication systems are being deployed in configurations that require the optical fiber to be tightly bent. This need has led to the development of optical fibers that utilize a ring of small, non-periodically disposed voids that surround the core region. The void containing ring serves to increase the bend insensitivity—that is to say, the fiber can have a smaller bend radius without suffering a significant change in the attenuation of the optical signal propagating in the fiber. Optical losses are minimized by placing the void containing ring region in the cladding of the optical fiber (some distance from the core); thus, the amount of light propagating through void containing ring region is minimized.
Because optical fibers are typically designed to efficiently deliver light from one end of the fiber to the other end of the fiber over long distances, very little light escapes from the sides of the typical fiber, and, therefore optical fibers are not considered to be well-suited for use in forming an extended illumination source. Yet, there are a number of applications such as special lighting, signage, or biological applications, including bacteria growth and the production of photo-bioenergy and biomass fuels, where select amounts of light need to be provided in an efficient manner to the specified areas. For biomass growth there is a need to develop processes that convert light energy into biomass-based fuels. For special lighting the light source needs to be thin, flexible, and easily modified to variety of different shapes.
Light diffusing fibers are important for applications such as specialty lighting, signage and display applications where selected amounts of light are required to be provided to the specified areas in an efficient manner. In one approach that was considered, nano-engineered features were formed in optical fibers to implement scattering centers configured to provide very efficient scattering of light through the sides of the optical fiber. The optical fibers and the scattering mechanisms formed in the fiber provide a very small, flexible illumination source. These optical fibers can also be bundled together to effectively increase the core size in order to more effectively couple light from an LED or similar light sources. The extraction of light from the fiber is generally uniform and may be tuned to scatter more or less light through the sides by controlling the number of scattering sites within the fiber.
Light diffusing fibers with scattering centers both in the core and the clad have been disclosed. In some embodiments, the light diffusing fiber comprises a silica core in which a section of the core contains a ring of non-periodically distributed (radially and axially) nano-engineered features acting as scattering sites. The scattering sites have diameters in the ˜50-500 nm range and lengths of ˜10-1000 mm. Since the scattering centers range in size from 50-500 nm, they effectively scatter the propagating light almost independent of the wavelength of the light used. The magnitude of scattered light is controlled by exploiting its dependence on the size of the scattering centers and their relative area compared to the fiber core. The absorption losses within the fiber are negligible, and the scattering losses can be as high as 5-10 dB/m. The clad of the fiber can be either F-doped silica or low index polymer clad, giving NA of the fiber up to 0.53. The bending losses are also small with minimum bending diameters as small as a 5 mm radius. One of the issues with aforementioned optical fibers relates to type of optical connectors that are suitable for use with a particular optical fiber.
For example, the F-doped silica clad fibers can be configured to be compatible with conventional fiber optic connector technologies. Stated differently, because the glass cladding is intimately connected to the ceramic ferrule, the strain relief of the fiber to the ferrule is relatively high and a high core to ferrule concentricity is more easily achieved. However, the NA achievable for conventional F-doped silica clad fibers does not allow for uniform illumination when the fiber is under bend. On the other hand, it is more problematic to use conventional fiber optic connector technologies with optical fibers that feature a low index polymer cladding. Optical fibers with low index polymer cladding must be installed with the connector with the cladding intact; and connectors of this type are commonly referred to as “crimp and cleave” fiber connectors. To be specific, the connector components are directly crimped onto the relatively soft polymer coating of the fiber, but the connector is not able to achieve physical contact because the fiber enfaces flatten against each other. Moreover, the strain relief of the fiber to the ferrule is much lower. Finally, high core to ferrule concentricity is not easily achieved due to imprecise low index coating thickness and centering. Briefly stated, when comparing polymer clad fibers to glass clad fibers, the polymer clad fibers achieve inferior results. Accordingly, the cost of terminating polymer clad fiber is relatively high because it cannot use a standard optical fiber ferrule connector.
What is needed therefore is an F-doped silica clad fiber that can be employed with a standard optical fiber ferrule connector while providing relatively uniform illumination in regions under bend.